Two-way radio communication system



April 1 H. A. ROBINSON TWO-WAY RADIO COMMUNICATION SYSTEM Filed June 29, 1944 2 Sheets-Sheet 1 nununnuu- N n n W m mog Ap 29', 'H. A. ROBINSON 2,419,593

TWO-WAY RADIO COMMUNICATION SYSTEM Filed Jime '29, 1944 '2 Sheets-Sheet 2 1'= 600 TO 900 KC.

FREQUENQY CHANNEL MC,-

600 TO 900 Kc.

5.5 as 6.1-6.4 6.7 7.0 .75 5.2 as as 9.1

FREQUENCY CHANNEL MC.

ZROBINSON.

ATTO R N EY.

Patente d Apr. 29, 1947 TWO-WAY RADIO COMMUNICATION SYSTEM Harris A. Robinson, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application June 29, 1944, Serial No. 542,746

9 Claims. 1

The present invention relates in general to a radio communication system, and more particularly to an improved system for two-Way communication which is capable of selective operation on any one of a multiplicity of frequency channels contained in a wide frequency spectrum.

In carrying out the invention there is employed at each of a multiplicity of points or stations a combination receiver-transmitter which is arranged to both receive and transmit on the same carrier frequency. A system of this general type is disclosed in Young Patent No. 2,113,419, assigned to the same assignee as this application, but the number of frequency channels on which that system is capable of operating is limited to the specific number of crystals employed.

In accordance with the present invention, it is possible to secure a very high order of frequency stability, closely approaching that of crystal control, with the advantages and flexibility of being ableto set up the receiver-transmitter on a multiplicity of frequency channels, each of which may be selected at any desired point in a Wide frequency spectrum, without the necessity of having a specific crystal ground to each desired operating frequency.

Briefly, the present system employs a crystalcontrolled oscillator provided with a number of selectively operable crystals, each of which determines the stability of the receiver and transmitter on a plurality of frequency channels. The arrangement differs from the more conventional crystal-controlled transmitter-receiver circuits in that these crystals have certain frequencies which are not directly related to the final frequency of transmission or reception. Hence, this set of crystals need not be changed when setting .the equipment to operate on any selected frequency throughout the operating spectrum.

The ability to set the equipment to operate at any selected frequency without changing the crystals depends upon theuse of an intermediate frequency (IF) amplifier, variable in frequency over a small tuning range. An IF oscillator, ganged with this variably tuned IF amplifier, is mixedwith the frequency of the crystal oscillator, or a multiple thereof, to provide the operating requency for transmission. Thus, for each crystal a small tuningrange is obtained with a negligible loss in frequency stability in comparison with the crystal stability. By combining the small .tuning ranges obtained for each of a set of crystals complete coverage of the frequency spectrum is obtained. In obtaining this wide frequency coverage with a limited number of crystals, advantage is taken of the fact that the final frequency of transmission or reception can be the crystal frequency, or a multiple thereof, plus or minus the variable intermediate frequency.

It is therefore one of the main objects of the present invention to provide a highly 'flexible two-way communication system in which a multiplicity of operating channels is made available in a wide frequency spectrum with the use of a limited number of frequency controlling crystal elements.

A more specific object of the invention is to rovide in a combined receiver-transmitter an intermediate frequency amplifier in the receiver and an intermediate frequency oscillator in the transmitter which are variable each over the same limited frequency thereby making available an increased number of operating frequencies.

Other objects and advantages will become apparent from the following description when read in conjunction with the accompanying drawings in which:

Fig. l is a block diagram of a receiver-transmitter which constitutes one of a plurality of stations in the communication system; and

Figs. 2A and 2B show graphically the various portions of the frequency spectrum between 2.0 and 9.1 mc. at which a plurality of frequency controlling crystals identified by their operating frequencies are effective in a specific embodi ment of the invention.

Briefly, the method employed utilizes a selected one of 'a plurality of available frequencies as the carrier for transmission in one direction and the same carrier for transmission inv the other direction. Tuning the receiverat station 1, for example, to the carrier frequency transmitted from station 2 will simultaneously assure perfect tuning of the receiver at station 2 in response to the carrier which will then be effective at station 1. This method is of particular value when the system is applied to military uses, for communication between mobile units, as for example, armored tanks, aircraft, or between mobile and ground stations.

The transmitter-receiver shown schematically in Fig. 1 represents a single set or station, of which any desired number of similar or other construction may be utilized for carrying out the method of communication herein described. Also, it is essential to the successful operation of the present system that a net of several stations maintain an agreed-uponv common working frequency for a given period during which communication is desired. Although any desired fre- 3 quency range may be employed, the description that follows will be given for a transmitter-receiver set which was designed for a frequency range from 2 to 9.1 me. in two bands.

Referring now more particularly to Fig. l, the local oscillator voltage for the receiver and the master oscillator voltage for the transmitter are derived from a common oscillator l, the frequency of which is controlled by one of a plurality of crystals 2 which are individually switched into circuit by an 8-position switch 3. In a practical embodiment incorporating the present invention, a set of 8 crystals was employed to cover the above mentioned wide frequency band, the crystals being ground to their respective fundamental frequencies given in the charts of Figs. 2A and 2B. As will be shown later each crystal serves to control the operation of the system over severallimited, spaced-apart frequency ranges, the sum total of which occupy the entire frequency spectrum for which the apparatus was designed. I The receiver employed i of the superheterody'ne type and comprises a radio frequency (RF) amplifier 4, a first detector or converter stage 5, an intermediate frequency (IF) amplifier 8, a second detector i, and an audio frequency (AF) aniplifier 8, the output of which may be fed to a pair of earphones 9 or other reproducing device". The converter 5, which may be of the type disclosed in Smith Patent No. 2,323,250, is supplied through a switch Si in the R or receive position and through an oscillator-amplifier it with oscillations from the local oscillator l, the frequency of which is determined by the setting of the crystal selector switch 3. By means of a switch ll amplifier it is adapted to have included in its output one or the other of two coils l2 and l3 clifiriected respectively to the fixed contacts F and S, sothj'at for 'each position'of the switch ll either the fundamental or the second harmonic of theoperative crystal is available as the'heterodyne frequency.

The receiver. is of "conventional design except thatIF amplifier e is tunable through a limited range of frequencies which in the present instance is from 600 to '900 kc. Although, for simplicity amplifier 6 is shown as a single stage, in actua practice it may comprise several similar stages, each having its input and output circuits individually tuned by ganged adjustable ferro magnetic cores or ganged variable capacitors. Atfl l there is shown the outputcircuit of one stage in coupled relation with'the input circuit l 5 of a succeedingstage, magnetic'cores l6 and H being utilized for adjusting the tuning of the respective circuits.

The RF'amplifier dis provided with a tunable input or antenna circuit'an'd a tunable output circuit, only one of which is shown for simplicity, and the converter stage 5 is provided with a tunableinputcircuit, the several circuits being similar and each comprising a variable condenser 18 and one of two indu'ctances i9 and 2c which are connected respectively to fixed contacts A and B, A band switch 25 associated with'each of theitunable circuits is adapted when in contact with A to connect coil l9 in shunt across its associated variable condenser it to permit'the circuit to be tuned over one frequency band, and when in contact with B to connect coil 29 in shunt across the same condenser to permit the circuit to be tuned over a second band. A band switching arrangement of'this 'typ'e is disclosed in pauses et a1. Patent 'No. 2,024,816. For convenience the two bands are identified as bands 4 A and B, respectively, to correspond with the similarly designated contacts.

The several variable tuning condensers it are interconnected for unicontrol adjustment by suitable means represented by the dash line 22 and the several band switches 28 are interconnected for simultaneous operation by suitable means represented by dot-and-dash line 23. A pair of switches S2 and-S3 are adapted in their R or receive position to couple an antenna (ANT) to the input of RF amplifier 1i, and a switch S4 in the R or receive position is adapted to couple the output of converter 5 to the input of IF amplifier 6.

The carrier frequency for transmission is derived from a conventional mixer or converter stage 2s which may be similar to the converter stage 5 of the receiver and of oscillator-amplifier l0 and also the output of a low-frequency variable oscillator 25 through a switch S5 in the T or" transmit position. The frequency determining circuit 2c of this oscillator is Variable through the same frequency range as the receiver IF amplifier 6 by means of an adjustable ferro-magnetic core 2'3, although a variable capacitor may be used, if desired. Oscillator 25 may therefore be called appropriately an IF oscillator. The several cores it, l? of amplifier 6 and the core 21 of oscillator 25 are mechanically interconnected for unicontrol adjustment by suitable means represented by the dash line 28.

In transmission the mixer 2:2 feeds its output successively through RF amplifier 4, converter stage 5 which now operates as an amplifier, an intermediate power amplifier (IPA) 29, and a power amplifier (PA) 3%, the output of which is coupled to the antenna. through switch S2 in the T or transmit position. In the T or transmit position of switch Si the source of local oscillations is disconnected from the converter stage and there is connected a cathode biasing network 3] suitable to make this stage function as anRF amplifier, replacing the cathode biasing network (not shown) which it utilized for the stage to functionas a converter or mixer. The several switches Si to S5 are adapted forsimultaneous operation between their T and R positions by means of a transmit-receive relay, not shown, commonly used in transceivers.

The IPA- stage 29 is provided with tunable input and output circuits, only one of which is shown, and thePA stage 3% is provided with a tunable output circuit, the several'circuits being similar to those of the receiver stages 4 and 5, their component parts therefore being similarly identified. The variable condensers IS of stages 29 and 39 are unicontroll'ed with those of stages 4 and 5 by means of 22, and band switches 2| of'stages 29 and 3!! along with harmonic selector switch ll of oscillator-amplifier it are unicontrolled with the band switches 2i of stages 4 and 5 by means of 23;

Since in the operationof the system the-carrier frequency for both reception and transmising transmission, This constitutes the subject matter of an application, Serial No. 546,845, filed July 27,1944; in the name ofW. A.- Harris and assigned to the same assignee as this applicatio'n 'jPower amplifier 30 is plate and screen modulated in the conventional manner, as shown in Lawrence Patent No. 1,923,543, by a modulator 32 energized from an audio amplifier '33 to which there is connected a microphone 34. For clarity the signal path for reception is shown by the single arrows while the signal path for transmission is shown by the double arrows.

It is desirable for a two-way communication system of the type described to be able to tune both the receiver and transmitter at each station to one of a plurality of prearranged'operating frequencies, expeditiously and as accurately as possible. For this purpose there may be provided a motor-operated, automatic preselector mechanism designated 35 having a plurality of selector units, so designed that one unit adjusts the variable condensers l8, ganged together by means 22, to one of ten preselected positions, a second unit adjusting, for each of said positions, the band switches 2| and the harmonic selector switch H, ganged together by the means 23, to one or the other of their positions, a third unit adjusting the cores I6, I! and 21, ganged together by the means 28, to one of ten positions corresponding'to the respective adjustments of condensers l8, while a fourth unit is adapted to select the particular crystal, the crystal selection depending upon the portion of the frequency rangein which the desired operating frequency is located. One type of preselector mechanism that is suitable for this purpose is disclosed in the Boterweg et a1. Patent N0. 2,351,185.

Since it is possible with the fixed frequency of I each crystal (fundamental or harmonic) to effect variations of the transmitter carrier over a narrow tuning range by means of the variable oscillator 25 and a corresponding variation in the receiver IF frequency by means of the variable IF amplifier 6, coverage of a wide band of frequencies may be had with a. limited number of crystals. This is clearly shown from the charts of Figs. 2A and 2B, the former showing the frequency coverage from 2,0 to 5.6 mo. and the latter showing'the frequency coverage from 5.5 to 9.1 me. In computing the individual narrowfrequency ranges for the several crystals the following frequency relation is employed.

Fr=frequency of transmission Fe=frequency of reception Fo=oscillator frequency (crystal) F1= variably tuned IE osc. and IF amplifier frequency n=oscillator multiplier: 11:1, 2, etc.

Utilizing, for example, the fundamental frequency of the crystal ground to 3500 kc. and mixing this frequency with the frequencies 600 to 900 kc. from the variable oscillator 25, the resulting difference frequencies in the range from 2.6 to 2.9 mc. may be selected as'carrier frequencies for transmission by appropriately adjusting the tuned circuits of stages .4 and 5. In reception this frequency range of 2.6 to 2.9 mc., to which stages 4 and 5 of the receiver are already tuned, is heterodyned with the fixed frequency output of oscillator-amplifier II! to produce the narrow range of frequencies, taking the difference frequencies, through which IE amplifier 6 is tunsue; 1

However, in utilizing the sum frequencies of F0 and Fr there is obtained for the crystal having' the fundamental frequency of .3500 kc. the frequency coverage from 4.1 to 4.4 me. As shown in Fig. 2A, with the use of the several crystals having the fundamental frequencies of 2900 to 4700 kc. and selecting the resulting difference frequencies, the frequency coverage is from 2.0 to 4.1 me. By utilizing the same crystals and selecting the sum frequencies the coverage is extended to 5.6 mc.

In Fig. 2B the frequency coverage is shown for the several crystals to extend from 5.5 to 9.1 mc., the second harmonic of each crystal being utilized in this. case. The upper row of narrow ranges is obtained by selecting the difference frequencies (2F0-F1) and the lower row is obtained by selecting the sum frequenci (2Fo+Fz) each narrow range being 0.3 mc. wide with the ranges of the upper row alternating with those of the lower row.

It is possible to obtain a still wider frequency coverage with the use of a doubler interposed between the variable osillator 25 and the transmitter mixer 24 and deriving the fourth harmonic of each crystal in the output of the oscillatoramplifier l0. In this case there would be employed an IF amplifier tunable over the frequency range 1200 to 1800 kc. In a practical embodiment in which these features have been employed and with the use of two additional crystals having the fundamental frequencies of 2750 kc. and 3050 kc. the frequency coverage was extended to 20.6 me.

Although it has been specified above that the several communicating receiver-transmitter sets are of similar construction, for a satisfactory operation of the system they need not necessarily be so, since as will be understood by those skilled in the art, the system may be operable also between a receiver-transmitter embodying the present invention and one which i capable of operating on the same frequency, as for example, the receiver-transmitter disclosed in the above Young patent.

While I have shown and described a preferred embodiment of my invention, it will be understood that various modifications and changes will occur to those skilled in the art without departing from the spirit and scope of this invention. I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim is:

1. The method of two-way communication between a'pair of stations, at least one of which comprises a transmitter provided with a variable oscillator and a superheterodyne receiver provided with an intermediate frequency amplifier that is tunable over a limited frequency range, which consists in generating a fixed stable highfrequency, mixing said generated fixed frequency with said selected frequency from a variable oscillator adjustable to the same frequency as the intermediate frequency amplifier of the receiver to derive a resultant frequency which constitutes the carrier frequency for the transmitter, receiv,

ing such carrier frequency from the other communicating station, and mixing the received carrier frequency with the fixed frequency to provide a beat frequency to which the receiver intermediate frequency amplifier is tuned.

2, The method of providing two-way communication on the same operating frequency between a pair of stations, at least one of which plifier is tuned.

comprises a transmitter having a variable, relatively low frequency Oscillator and a superheterodyne receiver having a variable intermediate frequency amplifier, said-oscillator and amplifier being tunable in unison over a limited frequency range, which consists in generating a fixed stable high-frequency, mixing said generated fixed frequency with a selected frequency from the variable oscillator to derive a resultant frequency which constitutes the carrier frequency for the transmitter, receiving such carrier frequency from the other communicating station, and mixing the received carrier frequency with the generated fixed frequency to provide a beat frequency which is the frequency to which the receiver intermediate frequency amplifier is tuned.

3. A two-way communication system comprising a plurality of similar radio stations which are adapted to communicate with one another on the same carrier frequency, each station comprising a transmitter and a superheterodyne receiver, a fixed frequency oscillator common to both receiver and transmitter, means during reception for supplying the oscillator frequency to the first detector of the receiver, a variable frequency oscillator tunable over a limited range of frequencies, and means during transmission for mixing a predetermined frequency from the variable oscillator and the frequency of the fixed oscillator to obtain a resultant frequency which is utilized as the carrier frequency for the transmitter.

4. A two-way communication system comprising a plurality of similar radio stations which are adapted to communicate with one another on the same carrier frequency, each station comprising a transmitter and a superheterodyne receiver provided with a variable intermediate frequency amplifier that is tunable over a limited frequency range, a fixed frequency oscillator common to both receiver and transmitter, means during reception for supplying the oscillator frequency to the first detector of the receiver, a variable frequency oscillator ganged with the intermediate frequency amplifier and tunable over the same limited frequency range, means during transmission for mixing a predetermined frequency from the variable oscillator and the frequency of the fixed oscillator to obtain a resultant frequency which is utilized as the carrier frequency for the transmitter and which upon reception is mixed with the frequency of the fixed oscillator to produce in the intermediate frequency amplifier the same frequency as the predetermined frequency from the variableoscillator.

5. Two-way communication equipment adapted to communicate on thetsame carrierfrequency, andcomprising atransmitter and a superheterodyne receiver, said receiver including'an intermediate frequency amplifier that is tunable over .a limited frequency range,.a fixedfrequency oscillator common to both receiver and-transmitter,.a variable frequency oscillator tunabl'eiin unison with the receiver intermediate amplifier over the same limited frequency range, means during transmission formixing a predetermined frequency from the variable oscillator and the frequency of the fixed oscillator to obtain a resultant frequencywhich is utilized as-the carrier frequency for the transmitter, and'means during reception for mixing said carrier frequency from a remote-station with the frequency of the, fixed oscillator. .to obtain :a. resultant frequency to which the receiver intermediate frequencyam- 6. Two-way communication equipment com-V prisin a transmitter, a superheterodyne receiver provided withan intermediate frequency amplifier which is tunable through a predetermined frequency range, a fixed stable high-frequency oscillator serving as the local oscillator forthe receiver, a variable oscillator tunable through the same frequency range as the receiver intermediate frequency amplifier, means for simultaneously adjusting the receiver intermediate frequency amplifier and the variable oscillator to the same frequency, and means for mixing a selected frequency of the variable oscillator with the frequency of the fixed oscillator to provide a selected carrier frequency for the transmitter which carrier frequency upon being received at one station from a second station is mixed with the local oscillator frequency to provide a resultant frequency to which the intermediate frequency amplifier of the receiver at said one station is tuned.

7. Two-way communication equipment adapted for selective operation on one of a plurality of frequency channels which is the same for both transmission and reception, each station comprising a transmitter, a superheterodyne receiver provided with an intermediate frequency amplifier which is tunable through a limited frequency range, a variable oscillator tunable through said same limited frequency range, means for simultaneously adjusting said variable oscillator and said intermediate frequency amplifier to a predetermined frequency within said limited range, an oscillator having associated with it a plurality of piezo-electric crystals which are adapted for selective connection to conrol the oscillator at a different fixed frequency, said fixed frequency crystal-controlled oscillator adapted during reception to serve as the local oscillator for the receiver and adapted during transmission to be mixed with said predetermined frequency of the variable oscillator to provide a desired carrier frequency for the transmitter, meansfor mixing the frequencies of the fixed and variable oscillators, and means for selecting the desired carrier frequency from said mixed frequencies.

8. In two-way communication equipment adapted to communicate on a selected one of a plurality of frequency channels which are contained in a wide frequency spectrum, said selected frequency channel being the same for both transmission and reception, equipment comprising a transmitter and a superheterodyne receiver, an

oscillator having associated with it a plurality of piezo-electric crystals which are adapted for selective connection to control the oscillator each at a difierent fixed frequency, each crystal determining a particular portion of the frequency spectrum in which the selected frequencychannel is located, an intermediate frequency amplifier included in the receiver tunable through a limitedzfrequency range, a variable oscillator tun-.-

able through the same limited frequency range,

means for simultaneously adjusting said variable oscillator and said intermediate frequency amplifier to a predetermined frequency within said limited range, said predetermined frequency determining the selected frequencychannel located within the particular portion of the frequency spectrum asdetermined by the operative crystal, said fixed frequency crystal-controlled oscillator adapted during reception to serve as the local oscillator for the receiver and adapted during transmission to be .mixed with said predetermined frequency of the variable oscillatqr to provide the selected carrier frequency for the transmitter, means for mixing the frequencies of the fixed and variable oscillators, and means for selecting the desired carrier frequency from said mixed frequencies.

9. A combination receiver-transmitter for obtaining a high order of frequency stability, closely approaching that of crystal control with the advantages and flexibility of being able to set up the receiver and transmitter on a multiplicity of frequency channels, selected at any desired points in a wide frequency spectrum, without the necessity of having specific crystals for the selected frequencies; equipment comprising a superheterodyne receiver and associated transmitter, an oscillator having associated with it a definite set of piezo-electric crystals which are adapted for selective connection to control a stable high-frequency oscillator at a definite fixed frequency, each crystal frequency or multiple thereof, determining a particular portion of the overall frequency spectrum in which the selected frequency channel is located, an intermediate fre- 10 quency amplifier included in the receiver tunable through the limited range of frequencies by which each of the crystals in the set differs from the adjacent crystal frequencies, a variable oscillator tunable through the same limited frequency range, means for simultaneously adjusting said variable oscillator and said intermediate frequency amplifier to a predetermined frequency within said limited range, said predetermined frequency determining the selected signal frequency channel located within the particular portion of the frequency spectrum as determined by the operative crystal, said fixed frequency crystalcontrolled oscillator adapted during reception to serve as the local heterodyne oscillator for the receiver and adapted during transmission to be mixed with said predetermined frequency of the variable oscillator to provide the selected carrier frequency for the transmitter, and means for selecting the desired carrier frequency from said mixed frequencies.

HARRIS A. ROBINSON. 

